Trays and other such generally planar articles are frequently used for holding a wide variety of foods and meal service items. The food industry has adopted food serving units including such trays and tray covers for maintaining temperature and serving meals on a large scale, commercial basis. Because of the increasing demand for such food serving units, manufacturers have sought to develop trays that are low cost, rigid, durable, and able to withstand numerous cleanings. In particular, the industry has sought to develop a tray and tray cover that can chemically withstand abrasive cleaners and harsh rinsing aids which substantially shorten the useful life of trays.
The difficulties associated with developing acceptable trays and tray covers was compounded when the food service industry expressed a desired for insulated trays and tray covers that could serve hot and cold items simultaneously without sacrificing rigidity or integrity. The solution found by the industry was the use of rigid plastics (e.g., polycarbonate, ABS, polyester, and polystyrene) which were thermoformed into two parts. See, U.S. Pat. Nos. 3,799,386; 4,386,703; and 4,545,487. A self expanding foam was used to insulate the tray and tray cover compartments and chemically bond the top and bottom parts together. A typical polycarbonate tray or tray cover, generally referred to as a food serving unit was made with the following steps:
1. Thermoforming sheets of polycarbonate into top and bottom shells having a flat rim around the unit; PA0 2. Roughly reducing the size of the thermoformed shells by die-cutting each shell; PA0 3. Drilling vent holes into the top and/or bottom shells; PA0 4. Pouring a self expanding foam into one of the shells; PA0 5. Adding a reinforcing rim to either the top or bottom shell with a solvent adhesive; PA0 6. Applying a solvent adhesive to the top and bottom rims; PA0 7. Assembling the top and bottom shells; PA0 8. Clamping the shells together while the foam expands inside the unit and the solvent adhesive cures; and PA0 9. Manually removing the excess rim material with a router and sanding the routed seam to a smooth finish.
Initially, these polycarbonate trays/tray covers proved to be useful despite the labor intensive process required for their assembly. It became apparent, however, that the polycarbonate trays/tray covers were adversely reacting with certain chemicals used in commercial cleaning operations. The effects of the adverse reactions was apparent by the formation of stress cracks on the tray/tray cover, separation of the adhered rim joint, and ultimate failure of the tray.
Over the years, coated and modified polycarbonates have been developed to increase their resistance to chemical degradation from commercial cleaning operations. While such coated and modified polycarbonates extend the useful life of the trays and tray covers to some degree, better chemical resistance is still desirable and the cost of these polycarbonates is relatively high. Clearly, a better alternative to polycarbonate food serving units would be desirable to enhance the durability of the units, simplify their method of assembly, and reduce their material cost.
Polypropylene is one potentially useful plastic material that is inexpensive and has better chemical resistance to cleaning chemicals than polycarbonates. Polypropylene was suggested for trays in U.S. Pat. Nos. Re. 30,969; 3,532,237; 3,754,640; and 3,799,143. Unfortunately, the same chemical resistance that would make polypropylene desirable as a food serving unit also renders the parts of the unit difficult to join. Very few solvent adhesives will react with polypropylene, and those that do (gasoline, benzene, carbon tetrachloride, acetone, alcohol, and wet chlorine) are not practical.
Indeed, one of the few ways to obtain a polypropylene joint with integrity is by fusing the seam surfaces together with applied or induced heat and pressure. "Spin welding" is one way of inducing fusion heat as described in GB 949,338. Spin welding is not available, however, to articles having a noncircular shape, and traditional thermal fusion with heated irons or plates results in the need for substantial maintenance and cleaning of molten plastic.
Insulated trays have been recently manufactured from thermoformed sheets of polypropylene. The manufacturing technique used to assemble the polypropylene trays is similar to the above technique for assembling polycarbonate trays and, thus, has similar labor cost and waste problems. That is, since the top and bottom shells are made from flat sheets, material waste occurs in addition to the time consuming steps of die cutting, drilling vent holes, clamping the parts, and removing excess rim material. The thermoformed polypropylene rims are fused by using heated plates despite their need for periodic cleaning and maintenance.
Additional problems would be presented if traditional injection molding was used for making planar products from polypropylene. Polypropylene exhibits substantial shrinkage upon removal from a conventional injection molding operation. This shrinking action would cause warping in relatively large, planar parts (e.g. shells for food trays or food tray covers) and would prohibit the accurate joining of one shell to the other.
It would be desirable to have a tray, tray cover or similar, generally planar article that could be made of inexpensive, chemically resistant materials with a process that is less labor intensive than previous processes.
It would also be useful to have an insulated tray or tray cover that could be made of a wide variety of relatively flexible plastic materials while retaining a high overall rigidity and structural integrity over an extended period of time.
It would further be advantageous to have a method for making trays and tray covers that was inexpensive, would produce little waste material, and would not require an idle period waiting for solvent adhesives to cure.